[iOS] Bump up the cocoapods version (pytorch#41895)

Summary:
Pull Request resolved: pytorch#41895

### Summary

The iOS binary for 1.6.0 has been uploaded to AWS. This PR bumps up the version for cocoapods.

### Test Plan

- Check CI

Test Plan: Imported from OSS

Reviewed By: husthyc

Differential Revision: D22683787

Pulled By: xta0

fbshipit-source-id: bb95b670a7945d823d55e9c65b357765753f295a

Author: xta0

ios/LibTorch.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
     s.name             = 'LibTorch'
-    s.version          = '1.5.0'
+    s.version          = '1.6.0'
     s.authors          = 'PyTorch Team'
     s.license          = { :type => 'BSD' }
     s.homepage         = 'https://github.com/pytorch/pytorch'